Resin transfer molding is a closed mold, low pressure process applicable to the fabrication of complex, high performance composite articles of both large and small size. Several different resin transfer molding processes are well known to those skilled in the art. The process differs from various other molding processes in that a reinforcing material or preform such as glass fibers or other fiber reinforcement, is placed separately into a mold tool cavity. Resin is then injected under pressure into the mold cavity to combine with the preform to form a fiber reinforced resin composite product.
Typically, a pre-shaped fiber reinforced preform is positioned within a molding tool cavity that has been carefully cleaned and coated with a standard mold release material to prevent the finished, molded part from catastrophically bonding to the mold (i.e., in some cases, adhesion of the finished, molded part to the mold is so complete that separation is impossible to accomplish and both the part and the mold has to be scrapped). After the molding tool is closed, the mold is subsequently evacuated by pulling or applying a mechanical vacuum to the mold cavity. This vacuum is typically in the range of 29 inches of mercury or greater. A feed line connects the closed molding tool cavity to a supply of liquid resin. The resin is pumped or "transferred" into the cavity where it impregnates and envelopes the fiber reinforced preform and is subsequently cured. The cured or semi-cured product is then removed from the molding tool cavity.
The primary advantage of resin transfer molding resides in its capacity for high rate production. Although this process is widely known, the use of this molding process has not become widespread because of problems associated with the process. For example, use of the process has been hampered by the difficulties associated with stabilizing and de-bulking the dry composite preform and loading the same into or around a mold cavity or part.
In addition, conventional resin transfer molds utilize "take apart" mold halves that are assembled around the dry fabric preform. It is difficult to seal these molds adequately enough to achieve a high level of vacuum integrity because of the numerous metal-to-metal joints. Furthermore, intensive clean-up of the mold apparatus is required after molding and prior to reuse as a result of the infiltration and intrusive properties of the low viscosity resins typically used in resin transfer molding. Not only does the resin completely coat all the visible mold surfaces, it further tends to penetrate, infiltrate and cement together adjacent mold surfaces that are inherently difficult to protect with standard mold release coatings. These surfaces would include crevices between mold sections, crevices between threaded upper/lower (or female/male) members, small clearances between close fitting alignment pins, and alignment bushings used to precisely assemble mold details to the mold body or provide alignment to the mold halves themselves during mold opening/closing sequences. The application of up to 500 degrees Fahrenheit (.degree.F.) to these molds during normal cycling tends to further aggravate the problem of achieving a vacuum tight mechanical fit as a reliable means of sealing the mold to prevent undesirable resin migration outside the molded part cavity area. Finally, handling and storage of the preforms is limited by the dry, loosely woven nature of the composite fabric. As a result, use of the process has typically been limited to simple low-strength components as compared to high strength aerospace components.
Thus, there is a continuing need in this field of art for means to obviate the above problems.